MRCP-0598

MRCP-0598

A 25-year-old woman was admitted to hospital with a 3 day history of feeling generally unwell, with fatigue, arthralgia and pruritis. She had recently finished a 5 day course of antibiotics for a urinary tract infection but there was no other significant past medical history. She had no significant findings on clinical examination except for a widespread erythematous rash. Investigation results are below: Haemoglobin (Hb) 12.6 g/dL, White cell count (WCC) 13.0 × 109/l (eosinophilia) Platelets 390 × 109/L, Creatinine 720 μmol/L, Na+ 135 mmol/L, K+ 5.2 mmol/L, Urea 22.0 mmol/L, Urinalysis Protein ++ blood + What is the most important investigation to establish the diagnosis?